arXiv subjects

C. Roiesnel

Publications and source records attributed to C. Roiesnel.

26 records · Page 2Linked to original sources

arXiv2001

Unquenched calculation of alpha_s from Green functions: progress report

We present preliminary results on the computation of the QCD running coupling constant in the MOM_tilde scheme and Landau gauge with two flavours of dynamical Wilson quarks. Gluon momenta range up to about 7 GeV (beta=5.6, 5.8 and 6.0) with a constant dynamical quark mass. This range already allows to exhibit some evidence for a sizeable 1/mu^2 correction to the asymptotic behaviour, as in the quenched approximation. We find Lambda_MS^(N_f=2) = 264(27) MeV x [a^{-1}(5.6,0.1560)/(2.19 GeV)], which leads to alpha_s(M_Z) = 0.113(3)(4). In view of the systematics error to be controlled, this encouraging result is more a preliminary indication than a real prediction.

hep-ph↗

arXiv2001

Asymptotic behaviour of the gluon propagator from lattice QCD

We study the flavorless gluon propagator in the Landau gauge from high statistics lattice calculations. Hypercubic artifacts are efficiently eliminated by taking the $\sum p_μ^4 \to 0$ limit. The propagator is fitted to the three-loops perturbative formula in an energy window ranging form $\sim$ 2.5 GeV up to $\sim$ 5.5 GeV. $α_s$ is extracted from the best fit in a continuous set of renormalisation schemes. The fits are very good, with a $χ^2$ per d.o.f smaller than 1. We propose a more stringent test of asymptotic scaling based on scheme independence of the resulting $Λ_{\bar MS}$. This method shows that asymptotic scaling at three loops is not reached by the gluon propagator although we use rather large energies. We are only able to obtain an effective flavorless three-loops estimate $Λ_{\bar MS}^{(3)}= 353 \pm 2 ^{+25}_{-10} $ MeV. We argue that the real asymptotic value for $Λ_{\bar MS}$ should plausibly be smaller.

hep-ph↗

arXiv2000

Lattice calculation of $1/p^2$ corrections to $α_s$ and of $Λ_{\rm {QCD}}$ in the $\widetilde{MOM}$ scheme

We report on very strong evidence of the occurrence of power terms in $\as(p)$, the QCD running coupling constant in the $\widetilde{MOM}$ scheme, by analyzing non-perturbative measurements from the lattice three-gluon vertex between 2.0 and 10.0 GeV at zero flavor. While putting forward the caveat that this definition of the coupling is a gauge dependent one, the general relevance of such an occurrence is discussed. We fit $Λ_{\bar{\rm MS}}^{(n_f=0)}= 237 \pm 3 ^{+ 0}_{-10}$ MeV in perfect agreement with the result obtained by the ALPHA group with a totally different method. The power correction to $\as(p)$ is fitted to $(0.63\pm 0.03 ^{+ 0.0}_{- 0.13}) {\rm GeV}^2/p^2$.

hep-ph↗

arXiv1999

Asymptotic scaling of the gluon propagtor on the lattice

We pursue the study of the high energy behaviour of the gluon propagator on the lattice in the Landau gauge in the flavorless case (n_f=0). It was shown in a precedin g paper that the gluon propagator did not reach three-loop asymptotic scaling at an energy scale as high as 5 GeV. Our present high statistics analysis includes also a simulation at $β=6.8$ ($a\simeq 0.03$ fm), which allows to reach $μ\simeq 10$ GeV. Special care has been devoted to the finite lattice-spacing artifacts as well as to the finite volume effects, the latter being acute at $β=6.8$ where the volume is bounded by technical limits. Our main conclusion is a strong evidence that the gluon propagator has reached three-loop asymptotic scaling, at $μ$ ranging from 5.6 GeV to 9.5 GeV. We buttress up this conclusion on several demanding criteria of asymptoticity, including scheme independence. Our fit in the 5.6 GeV to 9.5 GeV window yields $Λ^{\bar{\rm MS}} = 319 \pm 14 ^{+10}_{-20}$ MeV, in good agreement with our previous result, $Λ^{\bar{\rm MS}} = 295 \pm 20 $ MeV, obtained from the three gluon vertex, but it is significantly above the Schrödinger functional method estimate : $238 \pm 19$ MeV. The latter difference is not understood. Confirming our previous paper, we show that a fourth loop is necessary to fit the whole ($2.8 ÷9.5$) GeV energy window.

hep-ph↗

arXiv1999

Gluon propagator, triple gluon vertex and the QCD coupling constant

We study the UV-scaling of the flavorless gluon propagator in the Landau gauge in an energy window up to 9 GeV. Dominant hypercubic lattice artifacts are eliminated. A large set of renormalization schemes is used to test asymptotic scaling. We compare with our results obtained directly from the triple gluon vertex. We end-up with Λ_{\bar{\rm{MS}}} = 318(12)(5) MeV and 292(5)(15) MeV respectively for these two methods, compatible which each other but significantly above the Schrodinger method estimate.

hep-lat↗

arXiv1999

Three-loop beta function and non-perturbative $α_s$ in asymmetric momentum scheme

We determine the three-loop coefficient of the beta function in the asymmetric momentum subtraction scheme in Landau gauge. This scheme is convenient for lattice studies of α_s, the running coupling constant of QCD. We present high statistics lattice results for α_s in the SU(3) Yang-Mills theory without quark, compare with the three-loop running and extract the value of the corresponding Λ_\msbar parameter. We estimate the systematic error coming from four-loop terms. We obtain the result: Λ_\msbar = 295 (5) (15) {a^{-1}(β=6.0) \over 1.97 GeV} MeV.

hep-ph↗

arXiv1998

Lattice calculation of $α_s$ in momentum scheme

We compute the flavorless running coupling constant of QCD from the three gluon vertex in the (regularisation independent) momentum subtraction renormalisation scheme. This is performed on the lattice with high statistics. The expected color dependence of the Green functions is verified. There are significant $O(a^2μ^2)$ effects which can be consistently controlled. Scaling is demonstrated when the renormalisation scale is varied between 2.1 GeV and 3.85 GeV. Scaling when the lattice spacing is varied is very well satisfied. The resulting flavorless conventional two loop $Λ^{(c)}_{\bar{MS}}$ is estimated to be, respectively for the MOM and $\widetilde{MOM}$ scheme, 361(6) MeV and 345(6) MeV, while the three loop results are, depending on $β_2: Λ^{(3)}_{\bar{MS}} = (412 - 59 \frac{β_2} {β_{2, \bar{MS}}} \pm 6 MeV) $ and $Λ^{(3)}_{\bar{MS}} = (382- 46\frac{β_2} {β_{2, \bar{MS}}}\pm 5 MeV) $. A preliminary computation of $β_2$ in the $\widetilde{MOM}$ scheme leads to $Λ^{(3)}_{\bar{MS}} = 303(5)\frac {a^{-1}(β=6.0)} {1.97 GeV}$ MeV.

hep-ph↗

arXiv1995

Study of the Strongly Interacting Higgs Physics Model with Heavy Fermions

We study the effect of a possible fourth heavy generation of fermions on the Higgs sector of the standard model. We show, from the violation of elastic unitarity, that the scale of strong interactions is well below 1 TeV even with a Higgs mass as low as 500 GeV provided the fourth generation fermion mass is equal or larger than the Higgs mass. The diagonal Pade approximant method is then used to unitarize the partial wave amplitudes. It is found that, for the fourth generation fermion masses which are comparable to or larger than the Higgs mass, the Landau ghosts in the I=0 and I=2 channels of the reconstructed amplitudes move too close to the physical region to be accepted.

hep-ph↗